1. Field of the Invention
The present invention relates generally to the variable lift and timing of valves in internal combustion engines. More particularly, the invention concerns a system for controlling the opening and closing of the valves as well as for controlling the lift of the valves as a function of an engine speed.
2. Discussion of the Prior Art
In an internal combustion engine of the four stroke type, the phase of the engine cycle during which the intake and exhaust valves of the engine are open is usually referred to as valve timing. In a conventional engine the valve timing is typically fixed and does not vary with respect to engine speed. Since the dynamic behavior of the flow of gasoline into the cylinder varies considerably over the engine operating range, the fixed valve timing must, of necessity, be a compromise setting. In such a case, the fixed timing can be optimum only for a particular engine performance characteristic at a particular operating condition. For example, the fixed timing might be tailored for high output power at high speed at the expense of lower fuel consumption. Conversely, the fixed timing might be tailored to achieve minimum exhaust emissions or maximum fuel economy at the expense of power at high speed.
As is well understood by those skilled in the art of internal combustion engine design, an increase in power output can be achieved by increasing the opening time (duration) of the valves of the engine. Further, by increasing the extent of valve lift in proportion to duration, there will be less restriction in the valve area which will further enhance the filling and emptying of the cylinders. Accordingly, engines designed for racing require substantial valve opening duration and valve lift for peak performance. By way of example, the inlet valve in typical racing engines may open 80 degrees before top dead center and close 70 degrees after bottom dead center. Similarly, the exhaust valve may open 70 degrees before bottom dead center and close 50 degrees after top dead center. Longer duration for both inlet and exhaust is thereby achieved.
On the other hand, engines designed for normal passenger car use, which must be efficient at low speeds, are typically designed so that the inlet valves open at 25 degrees before top dead center, and close 45 degrees after bottom dead center. Similarly, the exhaust valves typically open 45 degrees before bottom dead center and close 30 degrees after top dead center thereby achieving a duration for both inlet and exhaust valves.
As is also well understood by those skilled in the art, late closing of inlet valves at higher engine revolutions per minute (rpm) will make better use of the kinetic energy in the incoming mixture of air and fuel. However, at lower rpm and at idle, some of the filling will be pushed back into the inlet channel. This condition creates increased emissions of hydrocarbon and fuel consumption, as well as uneven running. Power at low rpm is also decreased because of the loss of volumetric efficiency.
Additionally, the flow of exhaust gases through the exhaust valve is critical at the opening stage and benefits from longer durations at higher rpm. At lower rpm there is typically a loss of pressure energy by early opening that must be weighted against the loss at higher rpm caused by increased pressure meeting the piston during the exhaust stroke. Clearly, therefore, in actual practice, the engine designer must settle on some type of compromise between efficiency at either high or low engine speeds.
It is also well recognized that by permitting the closing of the exhaust valve to overlap the opening of the inlet valve, a significant increase in power can be accomplished. Therefore, by careful use of a large overlap of the exhaust valve, it is possible to make use of pressure pulsations in the sound waves that are created in both the inlet and exhaust systems. From the foregoing, it is apparent that the benefits of a progressive system that allows very small lift and very short duration of opening time also makes it possible to regulate the power output without strangling the effect of the throttle. By closing the inlet valve early, before bottom dead center, the compression cycle that follows will start from a lower level. This results in a lower peak combustion temperature and a reduction of nitrogen oxide emissions.
Because of the obvious drawbacks of fixed timing, and in light of the substantial advantages that can be realized by controlling valve lift and the opening duration of the valves of the engine, substantial effort has been directed in the past to the development of systems which will permit the timing to be varied as a function of engine operation. For example, U.S. Pat. No. 5,289,806 issued to Hurr, suggests the use of an axially shiftable camshaft to accomplish optimum valve opening and closing times for particular engine operating conditions. In a somewhat similar vein, U.S. Pat. No. 5,445,116 issued to Hara suggests a camshaft supported in an engine cylinder head structure, which has a low speed cam lobe and a high speed cam lobe which are used selectively to accomplish variable valve timing.
U. S. Pat. No. 5,404,770 issued to Kruger discloses a somewhat different approach. More particularly, Kruger suggests a variable cam arrangement for a lift valve which includes a cam having a rigid part which is movable on a camshaft in a radial direction between a retracted and an extended position. In another vein, the prior art Matsumo U.S. Pat. No. 5,078,102 discloses an engine valve driving device which includes a rocker arm attached at one end to a valve stem and carrying at its opposite end a roller which engages the surfaces of a stepped cam plate. The cam plate has a horizontal surface which includes upper and lower portions joined by an inclined portion so that horizontal reciprocating motion operates the rocker arm and the connected valve. The cam plate is connected to a rotary cam crankshaft so that rotation of the crankshaft produces linear reciprocating motion in the cam plate. The connection between the camshaft and the cam plate is adjustable, as by a hydraulic cylinder, to vary the gap between the cam plate and the camshaft and to thereby vary the timing of the motion of the valve with respect to the rotation of the camshaft.
Other prior art patents suggesting still other approaches to controlling valve timing include U. S. Pat. No. 5,551,021 issued to Moore; U. S. Pat. No. 5,361,736 issued to Phoenix et al and U. S. Pat. No. 5,483,930 issued to Moriya et al.
While the prior art attempts at designing systems for controlling valve duration and valve lift as a function of engine speed have met with some degree of success, numerous problems have been encountered in actually bringing the systems to commercial fruition. For example, many of the systems suggested in the past have been quite complicated and have exhibited marginal reliability in actual operation. Other prior art systems have been simply too difficult and costly to manufacture thereby making them unattractive to commercial engine developers.
As will be better appreciated from the description which follows, the apparatus of the present invention uniquely overcomes most of the drawbacks of the prior art and provides an elegant, extremely simple and highly cost effective means for effectively controlling valve lift and duration as a function of engine speed.